Efficient mixing of dissimilar fluids, or fluids and solids, is required in many situations including chemical processing, sewerage treatment, waste water aeration, etc. Mixing is typically achieved by rotating blades or impellers that physically displace the fluid thereby causing agitation that leads to mixing. The efficiency of this process is strongly dependent upon the design of the mixing blades and the nature of the components to be mixed.
U.S. Pat. No. 5,344,235 assigned to General Signal Corporation describes an improved impellor blade having an airfoil shape. The blade is coated with an erosion resistant material to overcome the problem of the blade deteriorating through use due to the impact with solids being mixed. The General Signal Corp patent is indicative of the general state of the art of impellor based mixers and aerators.
In assessing the prior art reference may be made to U.S. Pat. No. 4,865,459 assigned to Chuorika Co Ltd. This patent describes a mixer comprising a rotating disc mounted on a shaft a small distance from a fixed disc. A propeller is mounted on the end of the shaft for drawing in liquid which is forced through apertures in the rotating disc to be engaged by shear forces between the fixed and rotating discs. The shear forces between the discs is said to produce effective and uniform mixing.
Reference may also be made to U.S. Pat. No. 4,267,051 assigned to Rheintech Weiland. This patent describes an aerator comprising a group of parallel plates rotated near the surface of a liquid by a horizontal shaft. A rectangular cavity in the periphery of each disc traps air and forces it below the liquid level as the discs rotate. Trapping air and releasing it below the surface of liquid is known to be of only moderate efficiency for increasing oxygen content in the liquid. Although this device is likely to be more efficient than simply bubbling air through liquid, the improvement is likely to be small.
A more efficient aerator is described in French patent number 1580389 assigned to Societe Parisienne. This patent describes a hollow spindle having mounted at one end an air distributor comprising two parallel discs separated by four plates. A helix is mounted on the spindle above the air distributor to draw air into the apparatus for distribution through the liquid. Although this device would manage to force air into the liquid, bubbles will form and move immediately to the surface, thereby minimising the achievable benefit.
Despite the wide range of mixing apparatus designs and the extensive activity directed to improvements in aerators and mixers, there is still room for considerable improvement.